1. Field of the Invention
The present invention relates to an infrared sensor of bolometer type and a manufacturing method therefor.
2. Description of the Prior Art
An infrared sensor of bolometer type has detects infrared rays via a heat-sensitive layer which changes resistance according to temperature made of a metallic film and a ceramic such as vanadium oxide, or polycrystalline or amorphous silicon. When infrared rays irradiate the infrared sensor, the resistance of the heat-sensitive layer changes according to heat transferred from the adjacent absorption layer or the like. The change in resistance is detected as a change in voltage or current applied to the heat-sensitive layer, so as to detect irradiation of infrared rays.
The performance of bolometer type infrared sensor depends on the smallness of noise equivalent temperature difference (NETD). The NETD of the sensor is expressed as follows: EQU NETD.varies.G(1+.omega..sup.2 .tau..sup.2).sup.1/2 /(I.sub.b .alpha.R.sub.e .eta.), (1)
where I.sub.b denotes bias current through the heat-sensitive resistor layer, R.sub.e denotes resistance, .alpha. denotes temperature coefficient of resistance (TCR), .eta. denotes a ratio of absorption of infrared rays of the infrared sensor, G denotes thermal conductivity between the sensor and the substrate, .omega. denotes angular frequency of infrared rays, and .tau. denotes thermal response time.
A bolometer type infrared sensor satisfies following requirements: (1) The temperature coefficient of resistance (TCR) as detection sensitivity of infrared rays is large. (2) The thermal conductivity is low. (3) The thermal capacitance is small. (4) The ratio of absorption of infrared rays is large. That is, absorption and detection sensitivity of infrared rays are both large.
If polycrystalline or amorphous silicon is used as a heat-sensitive resistor layer, the sensitivity can be increased by implanting impurities of boron, phosphor, arsenic or the like to realize a resistivity and a high temperature coefficient of resistance (TCR) as desired. With respect to this point, a technique on controlling impurities in order to control the temperature coefficient of resistance is described for bolometers using polycrystalline or amorphous silicon as a heat-sensitive resistor, for example, in U.S. Pat. No. 5,021,663 and international application WO91/16607.
However, in the prior art infrared sensors comprising polycrystalline or amorphous silicon, if an amount to be added to realize a desired temperature coefficient of resistance is very small, an amount of infrared rays absorbed by the polycrystalline or amorphous silicon itself is very small.
Therefore, in order to enhance absorption of infrared rays, various structures of infrared sensor of bolometer type are proposed. For example, an infrared detector including a heat-sensitive semiconductor or resistor layer is formed above a cavity. A surface of the detector is almost covered with electrodes made of electrically conducting films for reading external signals. A heat due to absorption of infrared-rays in the electrodes is transferred to the detector to increase detection sensitivity of infrared rays.
In another modified structure, a heat absorption layer made of a metallic thin film is provided just above the infrared detector. Then, the heat absorbed by the layer is transferred to the infrared detected to improve detection sensitivity.
In a different modified structure, electrical leads for externally transmitting signals from the electrodes includes a metallic material such as TiN, and the leads also serve as a heat absorption layer. Then, it is a problem that an additional structure is needed which absorbs infrared rays by using a film other than the heat-sensitive semiconductor or the like to transfer the heat to the heat-sensitive semiconductor or the like, to detect the infrared rays efficiently.
Further, it is a problem that the resistance of the sensor depends of the state at the interface of the polycrystalline or amorphous silicon with the electrode and the kinds of the metal, its compound and the like. Especially, if the impurity concentration in the polycrystalline or amorphous silicon is low, it is difficult to get a reliable ohmic contact.
Further, if an infrared sensor has a smaller size, the sensors can be mounted at a high density. However, there is a problem that a decrease in an area for receiving infrared rays lowers the sensitivity.
An infrared image sensor includes a matrix array of the above-mentioned infrared sensors and selection circuits for determining scanning lines along vertical and horizontal directions. The scanning lines are connected to the sensors along rows and columns of the matrix array, and a switching element such as a field effect transistor is provided at each intersection of the scanning lines or for each sensor. Thus, it is a problem that a structure becomes complicated and that the selection circuits and the like occupy a large area so that mounting of the sensors at a high density becomes difficult.
Further, when a silicon integrated circuit and the infrared sensor are formed in the same production line, high temperature on forming the integrated circuit makes impurities diffuse from the high concentration impurity layers to an infrared detection section to deteriorate the detection sensitivity.
Further, in a prior art manufacturing method of infrared sensor, a material such as vanadium oxide or the like is used as a heat-sensitive resistor layer. Therefore, it is a problem that if a manufacturing apparatus for silicon semiconductor integrated circuit is used, the apparatus becomes dirty. Therefore, the sensor cannot be manufactured by using the same apparatus as the semiconductor integrated circuit. Thus, it is a problem that a yield of the infrared image sensor is worse so that it is expensive.
When a small infrared sensor is formed, it is a problem that scattering between lots arises due to a shift of the mask used in photolithography processes.